Leaf-atmosphere NH3 exchange of white clover (Trifolium repens L.) in relation to mineral N nutrition and symbiotic N2 fixation
2002
Herrmann, B. | Mattsson, M. | Fuhrer, J. | Schjoerring, J.K.
Plant-atmosphere NH3 exchange was studied in white clover (Trifolium repens L. cv. Seminole) growing in nutrient solution containing 0 (N2 based), 0.5 (low N) or 4.5 (high N) mM NO3(-). The aim was to show whether the NH3 exchange potential is influenced by the proportion of N2 fixation relative to NO3(-) supply. During the treatment, inhibition of N2 fixation by NO3(-) was followed by in situ determination of total nitrogenase activity (TNA), and stomatal NH3 compensation points (chi(NH3)) were calculated on the basis of apoplastic NH4(+) concentration ([NH4(+)]) and pH. Whole-plant NH3 exchange, transpiration and net CO2 exchange were continuously recorded with a controlled cuvette system. Although shoot total N concentration increased with the level of mineral N application, tissue and apoplastic [NH4(+)] as well as chi(NH3) were equal in the three treatments. In NH3-free air, net NH3 emission rates of < 1 nmol m(-2) s(-1) were observed in both high-N and N2-based plants. When plants were supplied with air containing 40 nmol mol(-1) NH3, the resulting net NH3 uptake was higher in plants which acquired N exclusively from symbiotic N2 fixation, compared to NO3(-) grown plants. The results indicate that symbiotic N2 fixation and mineral N acquisition in white clover are balanced with respect to the NH4(+) pool leading to equal chi(NH3) in plants growing with or without NO3(-). At atmospheric NH3 concentrations exceeding chi(NH3), the NH3 uptake rate is controlled by the N demand of the plants.
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